1. Field of the Invention
The present invention relates generally to rotary drum filters, and more particularly to an improved deck structure for such a filter.
2. Description of the Prior Art
Rotary drum filters are commonly used in the pulp and paper-making industry to separate wood pulp from its filtrate (see for example FIGS. 5 & 6). Such filters include a rotary drum partially submerged in a tank of pulp slurry. The drum has axially extending filtrate channels or compartments spaced about its periphery with such compartments being covered by a filter screen. The drum filter may be of the center valve type or the pipe machine type. With a center valve type filter each filtrate compartment opens into a center circumferential channel which communicates with a source of subatmospheric pressure. An arcuate, stationary valve member is mounted within and extends partially about the channel. As the drum rotates about its axis with subatmospheric pressure applied, the screen rotates through the pulp slurry, collecting a wet mat of fibers from the slurry. As the screen emerges from the tank, filtrate is drawn through the screen and compartments into the center channel, and from there is discharged through suitable piping, thereby removing the liquid from the mat. As the drum continues to rotate, the stationary valve member periodically blocks off the filtrate compartment outlets from the source of subatmospheric pressure, thus enabling removal of the pulp mat from the surface screen. A center valve type rotary drum filter is described in more detail in U.S. Pat. No. 4,154,687.
In a pipe machine type filter, the filtrate channels or compartments communicate through bucket sections, drainage conduits, and annular valve housing in communication with the source of the subatmospheric pressure. A stationary valve positioned within a valve seat portion of the valve housing controls the application of subatmospheric pressure to the respective valve chambers of the housing. As the drum rotates about its axis with subatmospheric pressure applied, the screen rotates through the pulp slurry and collects a wet mat of fibers from the slurry. As the screen emerges from the tank, filtrate is drawn through the screen and compartments into the bucket sections. From the bucket sections, the filtrate passes through drainage conduits and valve chambers, where it is discharged through suitable piping from the drum, and thus liquid is removed from the mat. As the drum continues its rotation, the stationary valve member periodically blocks off the filtrate compartments from the source of subatmospheric pressure, thereby enabling removal of the pulp mat from the surface screen. A conventional pipe machine type rotary drum filter is described in more detail in U.S. Pat. No. 3,363,774, and in U.S. Pat. No. 4,383,877 which is assigned to the assignee of the present application.
The described drum filters operate to lay a mat of wood pulp onto the surface of the drum as the pulp slurry is filtered through the screen. If desired, the pulp mat can be washed by directing showers at the mat prior to its removal from the drum.
One of the more important characteristics of drum filters is that they be able to rapidly remove water and other liquid from the passages adjacent the pulp mat. This is desirable because liquid being drawn through the mat has to pass through the covering screen into the filtrate compartments, where it is directed axially, under the pulp mat, for distances up to as much as 20 feet before being directed radially inward toward the center of the drum. Slow movement of this filtrate results in inefficient use of the subatmospheric pressure and is partially responsible for causing incomplete drainage of liquid from the pulp mat. This slow movement of filtrate, combined with the effects of gravity, also often causes rewetting of the pulp mat as a result of drainage from the filtrate compartments, back into the mat. This occurs primarily between the twelve and three o'clock positions (with a clockwise-rotating drum), and is most pronounced when the subatmospheric pressure is released for mat removal.
There are several drawbacks with incomplete drainage from the pulp mat. First, removal of liquid between the washing stages is incomplete, thereby decreasing the efficiency of the washing operations. Second, incomplete drainage often renders subsequent operations on the mat more difficult, for example, a greater quantity of chemicals, such as bleaches, caustic, etc., must be used in the next stages of treatment if a higher proportion of liquid remains in the mat as it leaves the drum. Third, incomplete drainage may result in the mat sticking to the drum at the mat discharge station, even though the vacuum has been released. This occurs because of the inability of sufficient air to pass through the relatively wet mat to relieve the vacuum beneath the mat. This sticking may very well result in tearing of the mat at the discharge station, since a heavy, liquid-loaded mat is often weaker that one which has been thoroughly drained.
Another drawback with many conventional designs is that after the drum surface passes the mat discharge station and prior to submergence in the slurry, liquid remaining in the filtrate compartments and the drainage conduits drains by gravity out through the filter screen, causing the compartments and conduits to be empty at resubmergence. This is undesirable because the air trapped in the conduits at resubmergence prevents the vacuum system from rapidly reapplying vacuum to the drum, thus interfering with accretion of the new mat on the drum. Trapped air in the filtrate compartments also interferes with the flow of filtrate through the drainage conduits during the formation of the mat.
In order to overcome the afore-described problems, attempts have been made to reduce the dimensions of the slots which open into the filtrate compartments. One such approach, exemplified in U.S. Pat. Nos. 3,680,708, 3,363,774, and 3,306,457, and depicted in FIG. 1, has been to utilize a filtrate compartment cover 2, one of which extends circumferentially from each of a plurality of axially extending filtrate dams 4, in a direction opposed to the direction of rotation of the drum, thereby defining narrow filtrate slots 6. A second such approach, exemplified by U.S. Pat. No. 3,794,178, and depicted in FIG. 2, has been to use a plurality of filtrate compartment covers 8 which extend in each circumferential direction to define narrow filtrate slots 9. U.S. Pat. No. 3,894,899 also discloses a system with relatively small slots into the filtrate compartments.
As a result of the filtrate compartment cover designs described above, these prior art proposals are some what effective in reducing the outward flow of filtrate from the filtrate compartment, thereby reducing rewetting. However, because of the substantial restriction on the free flow of liquid from the mat into the filtrate compartments, brought about by the reduced dimensions of the openings into the filtrate compartments, these prior art designs also tend to reduce the amount of liquid which is initially drained from the mat. The substantially closed construction of this type of design also retards the flow of filtrate along the filtrate compartments because of the increased surface friction. This tends to promote rather than retard rewetting.
Finally, in U.S. Pat. No. 4,370,231, which is assigned to the assignee of the present application, and which is incorporated herein by reference, a deck structure for a filtering apparatus including a rotatably mounted, generally cylindrical drum with filtering means and vacuum drainage means associated therewith. The deck structure includes a plurality of circumferentially spaced, axially extending, and radially projecting imperforate members affixed to the outer peripheral surface of the drum. The members each include a circumferentially extending surface which is radially spaced from the peripheral surface of the drum. The first surfaces define filtrate slots which are at least as wide in a circumferential direction as the first surfaces. A plurality of axially extending, evenly circumferentially spaced dams are mounted adjacent the outer periphery of no less than every second member (i.e., one dam for two or more members). The dams define filtrate compartments therebetween which lead to the vacuum drainage means within the drum. At least two subcompartments are defined by adjacent members within each filtrate compartment. Also included are perforate support means for supporting the filtering means above the filtrate compartments.